The present invention relates to a positioning control apparatus suited for positioning accurately a magnetic head in a magnetic disk apparatus.
In the magnetic disk apparatus to be employed as an external storage unit of a computer, a magnetic head is caused to follow a target track on a magnetic disk surface for performing recording/reproduction of data on/from the track. To this end, it is required to control the magnetic head so that it can be positioned on the target track with high accuracy. In recent years, there has arisen a trend of decreasing the track width in an effort for increasing the storage capacity of the magnetic disk apparatus. Under the circumstances, it is demanded to enhance or improve further the positioning accuracy as well as the seeking speed of the magnetic head.
As one of the factors of degrading the positioning accuracy, there can be mentioned vibration of a position signal making appearance in synchronism with rotation of the disk and external disturbance. Such vibration of the position signal has to be suppressed sufficiently and thus the control for suppressing adequately such vibration is required. As a technique to cope with this problem, there can be mentioned the one disclosed, for example, in JP-A-5-298842. This publication discloses a technique for positioning the magnetic head by following up eccentricity or decentering components contained in the position information. More specifically, according to the technique disclosed in this publication, a control signal is generated with the aid of a mathematical model having a resonance point at a vibration or oscillation frequency ascribable to the disk rotation, disturbance, etc. for suppressing the vibration, which control signal is then applied or added to a servo control signal for positioning the magnetic head.
With the technique disclosed in JP-A-5-298842 cited above, it is certainly possible to suppress the vibration components accompanying the rotation of the disk. However, when the frequency of the vibration lies within a frequency range in which the sensitivity function of the control system exceeds 0 (zero) dB, the control becomes unstable to undergo divergence, giving rise to a problem.